The goal of this research is to define the morphology of cell mediated immunologic reactions in man using improved (l micron sections) light microscopic methods as well as electron-microscopic and fluorescent antibody techniques. The study is based on a foundation of data which has established that delayed hypersensitivity (DH) reactions in man to contact allergens and protein antigens are characterized by infiltration of basophilic leukocytes, activation of the clotting system with fibrin deposition, and by heretofore undescribed hypertrophic-obliterative lesions of vessels cuffed by lymphocytes. Baseophils participating in these reactions undergo degranulation by means of a newly described mechanism, involving vesicular transport of granule substance to the cell surface. Mast cells also degranulate and at later intervals undergo mitotic proliferation. Based on these experiments, we are now prepared to study the morphology of biologically more complex, and medically more important, examples of cellular immunity in man. These include delayed reactions in individuals passively sensitized with transfer factor, skin allograft rejection, and the spontaneous immune response to a malignant tumor. Other experiments are directed at elucidating the significance of histamine, fibrin deposition, and the cutaneous fibrinolytic mechanism in DH. Finally, studies with purified basophil preparations will determine the immunospecificity of these cells in DH and the chemical contents of their cytoplasmic granules.